(1) Field of the Invention
This invention relates to conductive polymers and, more particularly, to conductive loaded resin-based materials for molding comprising micron conductive powders, micron conductive fibers, or a combination thereof, homogenized within a base resin when molded. Even more particularly, this invention relates to a moldable capsule, and a method for forming such a moldable capsule, wherein this moldable capsule is useful for molding a conductive articles usable within the EMF or electronic spectrums.
(2) Description of the Prior Art
Resin-based polymer materials are used for the manufacture of a wide array of articles. These polymer materials combine many outstanding characteristics, such as excellent strength to weight ratio, corrosion resistance, electrical isolation, and the like, with an ease of manufacture using a variety of well-established molding processes. Many resin-based polymer materials have been introduced into the market to provide useful combinations of characteristics.
In a typical scenario, these resin-based polymer materials are manufactured in bulk quantities by a chemical manufacturer as a raw material. This raw material is then sold to a molding operation where it is molded into particular articles. This raw material form of the resin-based polymer material typically comprises a plurality of small pieces called pellets or granules. These pellets are typically of uniform size, shape, and chemical constituency. At the molding operation, the pellets are loaded into a molding apparatus, such as an injection molding machine or an extrusion machine. The pellets are typically processed through a heating and mixing process in the apparatus where the material is converted from the solid state into the molten state prior to molding.
Most resin-based polymer materials are poor conductors of thermal and electrical energy. This characteristic is advantageously used in many applications. For example, the handles of metal cooking pans are frequently covered by a molded polymer material to provide a cool handling point for the heated pan. Many electrical interfaces, such as light switches, use resin-based polymers to prevent electrical exposure to the operator. This characteristic can be disadvantageous, however, in extending the use of resin-based polymer materials to applications long dominated by metal materials. For example, it is desirable to reduce weight of electrical and electronic circuit components used in airplanes. These components frequently comprise electrically conductive materials, such as copper, that add substantial weight to an airplane. Replacement of copper with a resin-based material would reduce the weight of the component and, by extension, the entire airplane. Unfortunately, most resin-based materials are not electrically conductive enough to be used as conductors.
Attempts have been made in the art to create intrinsically and non-intrinsically conductive resin-based materials. Intrinsically conductive resin-based materials incorporate molecular structures into the polymer to increase the conductivity of the material. Unfortunately, intrinsically conductive resin-based materials are expensive and provide only limited increases in conductivity. Non-intrinsically conductive resin-based materials are formed by incorporating conductive fillers into the base resin material to impute an increased conductivity to the composite material. Metallic and non-metallic fillers have been demonstrated in the art to provide substantially increased conductivity in the composite material.
Conductive resin-based materials formed using conductive fillers, as currently formulated in the art, suffer from several well-known problems. First, to achieve a high conductivity (low resistivity), a large amount of conductive filler may have to be used. However, as the amount of conductive filler increases, substantial molding problems can occur. For example, rapid wearing of molding machine components can occur. In addition, the desired material properties of the resin-based material, such as durability and ease of molding, may be sacrificed. An additional and very important problem may occur during the melting/mixing phase. To achieve a molded article having predictable performance characteristics, the molten resin-based material containing the conductive filler must be carefully mixed such that a consistent amount of filler is present throughout the molten mixture. Intuitively, this can be achieved through longer or more aggressive mixing processes. However, to achieve optimal performance, it is preferred that the filler comprises very small dimensions on the micron scale. Unfortunately, this type of filler can easily be destroyed, broken, or pulverized by overly aggressive mixing. As a further complication, the typical approach in the art is to load the molding apparatus with the resin-based material and the filler as separate components. That is, pure plastic pellets and filler material are loaded into a molding mixing apparatus as separate components and then mixed together. A dry (unheated) mixing may first be performed followed by a wet (heated) mixing to achieve a molten state. It is very difficult to achieve a homogeneous mix using this prior art process without resorting to overly aggressive mixing and experiencing damaged conductive filler components.
Recently, attempts have been made in the art to combine a filler and a resin-based polymer into a single pellet. However, these composite pellets are found to create several problems. First, these pellets are essentially formulated to be convenient carriers for the conductive filler. Typically, a resin-based material is impregnated between strands or pieces of conductive filler to adhere the pieces together. A second layer, or perhaps several layers, of resin-based material are then formed over the strands or pieces to complete the pellet. Typically, composite pellets of this type found in the prior art are formulated based on percent volumes of conductive material and of plastic. However, it is found that this pellet contains only a relatively small amount, by weight, of resin-based material when compared to the amount, by weight, of filler. Typically, these composite pellets are manufactured with filler content, by weight, of greater than 90%. This pellet, therefore, does not provide anywhere near a sufficient amount of resin-based material for successfully molding an article. Therefore, when a quantity of these composite pellets is loaded into the molding mixer, an additional quantity of pure (non-filler containing) resin-based pellets must also be loaded to provide the bulk material for molding. This mixture of composite filler pellets and pure plastic pellets forms a “salt and pepper” mix of pellets that must be carefully mixed and melted prior to molding.
This “salt and pepper mix” of composite filler pellets and pure plastic pellets creates several problems. First, it is very difficult, if not impossible, to create a homogeneous mixing of the filler material throughout the molten plastic. The resin-based material surrounding the composite pellets is designed to be relatively thin and to have a lower melting point that the bulk plastic pellet material into which it is mixed. These design features are intended to allow the composite pellets to quickly release the filler material into the surrounding pure plastic pellets. However, this approach is found to be counterproductive in practice. It is found that an early release of the conductive filler increases the amount of filler breakage during mixing. In addition, unless the entire mixture is over mixed to the point of destroying the fiber structures, it is difficult, if not impossible, to achieve a homogeneous mixture. The fibers tend to gang together, to create swirls, balls, or hot spots within the mixture. If the molten mixture is over-mixed, the destruction of the fiber structures dramatically reduces the conductivity of the molded article and eliminates many of it benefits. If the molten mixture is under-mixed to protect the fiber structure, then the poor homogenization as described above will result in a molded article of very unpredictable qualities.
Further, the composite pellet and the pure plastic pellet both contain resin-based materials. It is found that any dissimilarities in the chemical properties of the actual materials used in each of the types of pellets will result in further poor homogenization and in unpredictable properties in the molded article. Generally, it is found that a very electrically inconsistent, unstable, structurally weakened, and/or poor quality article is molded when using this “salt and pepper” mixing of pellets. It is a primary objective of the present invention to provide a new molding formulation based on a moldable capsule with improved molding performance and molded article characteristics.
Several prior art inventions relate to conductive plastic materials, methods of manufacture, and articles of manufacture. For example, U.S. Pat. No. 5,397,608 and U.S. Pat. No. 4,664,971 to Soens each teach a process for manufacturing a plastic article containing electrically conductive fibers. The process taught comprises drawing a bundle of stainless steel filaments through a polyester solution, drying, impregnating (through extrusion) more of the same polyester, cutting into granules, dry mixing with thermoplastic pellets, extruding again, cutting again into pellets, dry mixing with pure plastic pellets, and molding the item. A fiber/plastic granule described has a conductive fiber content ranging from about 30% to 70% by volume (U.S. Pat. No. 5,397,608 to Soens, col. 4, lines 1–4). Based on typical resin specific gravity ranging between about 1.0 and 2.0 and typical stainless steel specific gravity of about 7.9, the above-cited volumetric-based range translates to between about 63% and 95% fiber content by weight for the granules. Additional sub-product versions of the fiber/plastic granules are described as having fiber content by weight of 93.8% (col. 6, lines 15–17), and having fiber content by weight of 87% (col. 6, lines 23–26), and having a fiber content by weight of 8% (col. 6, line 36). Molded articles are described having fiber content by weight of 4% (col. 7, line 20). This art teaches fiber/plastic granules with relatively high fiber content by weight (above 60%) that are mixed with a large amount of pure plastic prior to molding articles with relatively low fiber content by weight (less than 10%).
U.S. Pat. No. 4,788,104 to Adriaensen et al teaches the manufacture of a granular composite containing crimped stainless steel fibers for use in the injection molding of plastic articles with shielding properties against electromagnetic radiation. The process involves the steps of forming a granular composite of gear crimped stainless steel filaments embedded into a linear polyester resin and coated with a modified alkyd resin and chopped into granules. These granules are then dry mixed with another base resin granule and then extruded and chopped to form other granules that can be mixed with pure plastic to form articles. The granules are described as having fiber content by volume of between 20% and 80% (col. 3, lines 61–65). This content translates to fiber content by weight of between about 50% and about 97% based on typical resin specific gravity ranging between about 1.0 and 2.0 and typical stainless steel specific gravity of about 7.9. Exemplary articles manufactured from this material have a fiber content of about 10% by weight (col. 4, lines 49–52).
U.S. Pat. No. 6,455,143 B1 to Ishibashi et al teaches a fiber reinforced thermoplastic resin composition that has good flowability during the molding process and allows the fibers to be well dispersed in the molded product. This patent teaches the use of fibers having a high strength and elastic modulus such as carbon fibers, glass fibers, polyaramid fibers, alumina fibers, silicon carbide fibers or boron fibers for improving the mechanical properties of the molded product.
U.S. Patent Publication US 2003/0089892 A1 to Fox et al teaches an electrically conductive thermoplastic polymer composition which comprises a combination of metal fibers and metal-coated fibers. The metal-coated fibers taught in this invention are typically a non-metallic fiber such as a carbon, glass or a polymer core with a coating of silver, nickel, aluminum, chrome, tin, or lead.
U.S. Patent Publication US 2003/0111647 A1 to Rosenzweig teaches electrically conductive polymeric composites where the filler material is a combination of stainless steel that is plated with tin or a tin alloy. In this invention tin plated stainless steel fiber is cut into pellets which are then mixed with resin granules and extruded to form a conductive plastic article. The melting point of the resin is higher than that of the tin or tin alloy such that the tin plating melts during the molding operation to form conductive connections between stainless steel fibers in the final matrix. No content percentages are given.
U.S. Pat. No. 4,960,642 to Kosuga et al teaches a method of manufacturing pellets for making electromagnetic wave shielding material. In this invention, the pellets are formed by impregnating a metal fiber with a first polymer via a first extrusion process, coating the metal fiber with a desired base resin via a second extrusion process, and then cutting into a pellet form. This reference teaches against greater than 30% resin content by weight for the pellets (col. 3, lines 50–60) and teaches against forming pellets using a single step process of extruding resin directly onto the fibers (col. 6, lines 26–37, and TABLES 1 and 2).
U.S. Pat. No. 5,525,423 to Liberman et al teaches a method of manufacturing a fiber tow having fibers of plural diameters encapsulated within a polymeric material to form a two dimensional conductive layer. This invention teaches the encapsulation of the fiber tow thru extrusion and subsequently cutting the extruded composite material into plugs. The invention then teaches mixing the composite plugs with other plastics in an injection molding process to form EMI shielding items.